Atomic Structure: Delving into Nucleus, Isotopes, and Radioactivity

Atomic Structure: Delving into Nucleus, Isotopes, and Radioactivity

Unit 18:


Understanding these concepts not only sheds light on the structure of matter but also helps us explore their impact on various scientific fields and everyday applications!

Atom Structure:

  • Atoms consist of a nucleus at the center and electrons moving around it.
  • The nucleus contains protons (positively charged) and neutrons (neutral).

Atomic Number and Atomic Mass Number:

  • Atomic number (Z) refers to the number of protons in the nucleus, identifying the element uniquely.
  • Atomic mass number (A) is the total count of protons and neutrons in the nucleus.

Isotopes and Radioactivity:

  • Isotopes are versions of an element with the same atomic number (same type of atom) but different atomic mass numbers (different number of neutrons).
  • Elements with an atomic number beyond 82 tend to be unstable and undergo natural radioactivity, changing into other elements.

Half-Life and Radioactive Elements:

  • The half-life of a radioactive element is the time required for half of its atoms to decay into other elements.
  • Rocks, soil, and water often contain radioactive elements that emit background radiation.

Applications of Radioactive Isotopes:

  • Radioactive isotopes have diverse applications in medicine (e.g., medical imaging and cancer treatment), agriculture (such as studying plant growth), and industry (like checking for leaks in pipelines).

Carbon Dating and Nuclear Processes:

  • Carbon dating estimates the age of once-living organisms by comparing the ratio of carbon-14 in living and dead samples.
  • Nuclear processes include nuclear transmutation, where heavy nuclei transform into lighter ones; nuclear fission, where a heavy nucleus splits into two parts; and fusion, where light nuclei combine to form a heavier nucleus.


i. Isotopes are atoms of the same element with different

  • (a) atomic mass (b) atomic number
  • (c) number of protons (d) number of electrons

ii. One of the isotopes of uranium is. The number of neutrons in this isotope is 238

  • 92U
  • (a) 92 (b) 146
  • (c) 238 (d) 330

iii. Which among the following radiations has more penetrating power?

  • (a) a beta particle (b) a gamma ray
  • (c) an alpha particle (d) all have the same penetrating ability

iv. What happens to the atomic number of an element which emits one alpha particle?

  • (a) increases by 1 (b) stay the same
  • (c) decreases by 2 (d) decreases by 1

v. The half-life of a certain isotope is 1 day. What is the quantity of the isotope after 2 days?

  • (a) one-half (b) one-quarter
  • (c) one-eighth (d) none of these

vi. When Uranium (92 protons) ejects a beta particle, how many protons will be in the remaining nucleus?

  • (a) 89 protons (b) 90 protons
  • (c) 91 protons (d) 93 protons

vii. Release of energy by the Sun is due to

  • (a) nuclear fission (b) nuclear fusion
  • (c) burning of gases (d) chemical reaction

viii. When a heavy nucleus splits into two lighter nuclei, the process would

  • (a) release nuclear energy (b) absorb nuclear energy
  • (c) release chemical energy (d) absorb chemical energy

ix. The reason carbon-dating works is that

  • (a) plants and animals are such strong emitters of carbon-14
  • (b) after a plant or animal dies, it stops taking in fresh carbon-14
  • (c) there is so much non-radioactive carbon dioxide in the air
  • (d) when plants or animals die. they absorb fresh carbon -14


18.1. Atomic Number vs. Atomic Mass Number:

The atomic number represents the number of protons in an atom’s nucleus, denoted symbolically as Z. The atomic mass number indicates the total number of protons and neutrons in an atom’s nucleus, symbolically represented as A. A nuclide is symbolically represented as ��XZA​X, where A is the atomic mass number, Z is the atomic number, and X represents the chemical symbol of the element.

18.2. Radioactivity and Elements:

Radioactivity is the spontaneous emission of particles or radiation from the unstable atomic nucleus of certain elements. Elements are radioactive if their nuclei are unstable due to an imbalance between the number of protons and neutrons. Elements with larger atomic numbers tend to be more unstable and, therefore, more likely to be radioactive.

18.3. Artificial Production of Radioactive Elements:

Radioactive elements can be produced artificially by bombarding stable nuclei with high-energy particles or by inducing nuclear reactions in a particle accelerator. For example, the element technetium (Tc) was first produced artificially by bombarding molybdenum (Mo) with neutrons.

18.4. Basic Radioactive Decay Processes:

The three basic radioactive decay processes are alpha decay, beta decay, and gamma decay. They differ in the particles emitted:

  • Alpha decay emits an alpha particle (composed of two protons and two neutrons).
  • Beta decay releases beta particles (electrons or positrons).
  • Gamma decay involves the emission of high-energy gamma rays without altering the atomic or mass number.

18.6. Atomic Number Increase in Nuclear Decay:

During nuclear decay, the atomic number can decrease but not increase. For instance, in beta decay, a neutron transforms into a proton, and an electron is emitted. An example is the decay of carbon-14 into nitrogen-14: 614C→714N+�−614​C→714​N+e

18.7. Half-life of a Radioactive Element:

The half-life is the time taken for half of the radioactive nuclei in a sample to decay. It is a characteristic property of a radioactive substance and helps predict the rate of decay.

18.8. Spontaneity of Radioactivity:

Radioactivity is a spontaneous process because it occurs without external influence or control. An experiment demonstrating this involves placing a radioactive source in a controlled environment and observing the emission of radiation without external stimulation.

18.9. Background Radiations and Sources:

Background radiations are constant low-level radiation present in the environment from various sources like cosmic rays, radon gas, terrestrial sources like rocks and soil, and human-made sources like nuclear fallout.

18.10. Uses of Radioisotopes:

  • Medicine: Radioisotopes are used in medical imaging (e.g., technetium-99m in nuclear medicine) and cancer treatments (e.g., iodine-131 for thyroid cancer).
  • Industry: They’re employed in industrial radiography to inspect welds and detect flaws in materials.

18.11. Radiation Hazards and Precautions:

  • Hazards: Health risks and environmental damage are common hazards.
  • Precautions: Safety measures include shielding, distance, and time limitation of exposure to minimize risks.

18.13. Nuclear Fusion vs. Fission:

Nuclear fusion is more sustainable than fission as it involves the combining of light atomic nuclei to release energy, producing less long-term radioactive waste and utilizing abundant fuel sources like hydrogen isotopes.

These explanations cover the provided questions regarding atomic structure, radioactivity, decay processes, applications, hazards, and sustainability of nuclear reactions.

18.1. Is it possible for an element to have different types of atoms? Explain.

Yes, an element can have different types of atoms known as isotopes. Isotopes of an element possess the same number of protons in their nucleus (which defines the element) but differ in their number of neutrons. This variance in neutrons results in isotopes with different atomic masses.

18.2. What nuclear reaction would release more energy, the fission reaction or the fusion reaction? Explain.

Fusion reactions release more energy per reaction than fission reactions. Fusion involves combining lighter atomic nuclei to form heavier ones, as seen in stars and hydrogen bombs, yielding vast amounts of energy. Fission, on the other hand, involves splitting heavier nuclei into lighter ones, utilized in nuclear reactors and atomic bombs, releasing considerable but comparatively less energy.

18.3. Which has more penetrating power, an alpha particle or a gamma ray photon?

Gamma-ray photons possess more penetrating power compared to alpha particles. Alpha particles consist of two protons and two neutrons, being relatively large and heavily charged, which results in their limited ability to penetrate materials. Gamma rays, however, are high-energy electromagnetic radiation with no charge or mass, allowing them to penetrate deeply through various substances.

18.4. What is the difference between natural and artificial radioactivity?

Natural radioactivity arises from the spontaneous decay of unstable atomic nuclei in naturally occurring elements. Artificial radioactivity, on the other hand, involves the creation of unstable nuclei through human activities, such as nuclear reactors or particle accelerators, which may not naturally occur or exist in stable isotopes.

18.5. How long would you likely have to wait to watch any sample of radioactive atoms completely decay?

The time needed for a radioactive substance to completely decay depends on its half-life—the duration it takes for half of the radioactive atoms in a sample to decay. For practical purposes, a sample will never completely decay, but after multiple half-lives, the remaining amount becomes infinitesimally small, making it effectively undetectable.

18.6. Which type of natural radioactivity leaves the number of protons and the number of neutrons in the nucleus unchanged?

Gamma decay or emission involves the release of gamma rays from an excited nucleus transitioning to a lower energy state. Unlike alpha or beta decay, gamma decay doesn’t affect the number of protons or neutrons in the nucleus, only releasing energy in the form of gamma rays.

18.7. How much of a 1 g sample of pure radioactive substance would be left undecayed after four half-lives?

After each half-life, half of the original radioactive material decays. After four half-lives, (12)4=116(21​)4=161​ of the original substance remains undecayed. Therefore, 116161​ of a 1 g sample would remain undecayed, which is 0.0625 grams.


In exploring the intricate world of atomic structure, we’ve uncovered the nucleus as the central hub housing protons and neutrons, defining an element’s charge number and atomic mass. The discovery of isotopes sheds light on the intriguing variations within the same element, while the realm of radioactive elements presents a fascinating journey into natural decay processes. The randomness of radioactivity transcends space and time, unraveling the enigmatic nature of atomic behavior, and guiding us deeper into the core of scientific discovery.